Big and little! Cells very in size depending on what they do. The female gamete is the only one you can see...it is the size of a period on a piece of paper. The male gamete is 100 times smaller. The sciatic nerve can be over a meter long, and runs from your back to your foot. Viruses generally are much smaller...less than any light microscope can see.

Why ,do U think , cells need to maintain size : surface area ration lower ?
I think for a particulas size of cell to carry out its functions properly it is essential to have certain value of surface area; it is essential for carrying out sufficient exchange of materials .

But think that volume change of something is in cube form (like m3, cm3,mm3 etc...) . But area (surface area) change is in square form. (like m2,cm2,mm2 etc...).
So they must have a proper ratio to maintain optimum transport.